How can I get the first selection of a layer in a QgsMapLayerComboBox?

Question

I'm developing a very simple QGIS plugin to calculate raster average by using GDAL. It uses a QgsMapLayerComboBox and a filter to select only raster layers. However, it needs to select twice a layer at the Combo Box to produce the expected results. The snipped code is:

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def initGui(self):
        """Create the menu entries and toolbar icons inside the QGIS GUI."""

        icon_path = ':/plugins/AverageRaster/AverageRaster.png'
        self.add_action(
            icon_path,
            text=self.tr(u'To calculate raster average'),
            callback=self.run,
            parent=self.iface.mainWindow())

        self.wcb = QgsMapLayerComboBox(self.dlg)
        self.wcb.setFixedWidth(220)
        self.wcb.move(140,40)
        self.wcb.setFilters( QgsMapLayerProxyModel.RasterLayer )
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def run(self):
    """Run method that performs all the real work"""
    #My code starts here
    layer = self.wcb.currentLayer()
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    self.dlg.show()
    # Run the dialog event loop
    result = self.dlg.exec_()
    # See if OK was pressed
    if result:
        # Do something useful here - delete the line containing pass and
        # substitute with your code.
        #My code starts here
        message = "the raster average of " + filename + " is = " + str(average) 
        QMessageBox.information(None, "Raster Average", message)
        #My code ends here

At the next image, my Plugin is determining the average of ndvi.tif raster. If I want the average of utah_demUTM2.tif, the expected value is obtained only at the second chance. How can I get the average at the first selection of a layer at the QgsMapLayerComboBox.

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Afterward that I tried out a lot of possibilities, I found a possible solution but, I don't like it very much because I have to click on icon plugin again and not on a OK button.

At the 'Property Editor' of Qt Designer, I unmarked the 'OK' button and I modified the label "Click on OK to get raster average" for this one: "Click on plugin icon again to get raster average".

After reloaded the plugin, I observed that the order in the Combo Box was alphabetic.

When I selected anyone of these layers (see above image) and I clicked on icon plugin, its behavior was always as expected and the principal Dialog never closed.

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I changed the QMessageBox for a QTextBrowser and put again the OK button at the widget Dialog. In this case, I could observe that the QgsMapLayerComboBox has the expected behavior when I select the raster layer. The problem is when I clicked on the OK buttom. The Dialog was closed and I need to launch the plugin again to see the results. It is possible to keep the window Dialog open when the OK button is clicked?

Answer 1

I tried this and it worked :

in your function run :

Put the line layer = self.wcb.currentLayer()

after if result:

It did what I think you wanted it to do.

On a side note : you know you can make your app calculate the average without clicking on ok? If you play with the signals, you can make a text appear in your main window each time you change selection.

Answer 2

This is the solution:

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    def __init__(self, iface):
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        okButton = self.dlg.pushButton
        okButton.connect(okButton, SIGNAL("clicked()"), self.myFunction)
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    def myFunction(self):
    """Run method that performs all the real work"""
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    def run(self):
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It was necessary to create a new Ok button and the old OK button, from button_box, it was hidden.